Along with the development of xDSL techniques, a series of xDSL standards related with xDSL techniques have been generated. A standard suit proposed by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) specifically includes; the G.992.1/2 about Asymmetrical Digital Subscriber Loop (ADSL), the G.993.1 about Very High Speed Digital Subscriber Loop (VDSL) etc. Moreover, each standard also defines some appendixes for satisfying different requirements of services or regions. The relatively outstanding one is ADSL2 annex L, which can be called READSL2 as well.
With respect to convenience of application, the ADSL2+ system needs to be compatible with the ADSL2 and the READSL2. VDSL2 devices support the VDSL2, the VDSL1, the ADSL2+, the ADSL2 and the READSL2 in the same modem.
As there are too many modes which can be selected, in most cases, both central office unit of xDSL (xTU-C) and remote unit of xDSL support a multiple mode, that is, they may support the ADSL2+ annex A, the ADSL2 annex A, the ADSL2 annex L ADSL annex A and the G.lite annex A in the same modem, or support the VDSL2 annex A, the ADSL2+ annex A, the ADSL2 annex A, the ADSL2 annex L ADSL annex A and the G.lite annex A in the same modem. Thus, before practical service communications are implemented, it is needed to select the best operating mode automatically between the xTU-C and the xTU-R.
In the xDSL network, in order to select a standard from numerous ones, i.e., in order to select an operating mode corresponding to a technical standard, the ITU-T defines a corresponding handshake protocol, namely G.hs. The xTU-C and the xTU-R transmit configuring parameters at the handshake stage, and negotiate with each other to accomplish selecting an operating mode for the xDSL network.
At present, there are two primary methods for selecting an operating mode automatically.
The first method is to select an operating mode which maximizes the sum of a speed of uplink and a speed of downlink.
The specific implementing procedure of the first method includes: at the handshake stage, the xTU-C and the xTU-R carry out a first training, namely, they select randomly a mode to implement training. At the channel analyzing stage of the training, the xTU-C and the xTU-R transmit a Medley signal respectively to enable the other side to measure a Signal Noise Ratio (SNR) and then they exchange their own measured SNRs. The xTU-R transmits a table composed of SNRs of each tone acquired at the Medley stage to an xTu-c. The xTU-C uses the received table to calculate a possible speed of uplink and a possible speed of downlink in various modes respectively. The xTU-C selects an operating mode which maximizes the sum of the speed of uplink and the speed of downlink according to the calculated results. If the selected mode is different from the current training mode, the xTU-C will restart training with the selected mode.
It can be seen that: although the first method can select exactly the xDSL network operating mode according with the requirements of practical services, it usually takes two times of training for the modems to acquire the exact mode. Training twice means longer training time than the time of normal operation, that is, it needs a long time to select an optimal operating mode. In this way, the time of the handshake between a central office unit and a remote unit may exceed the upper limit for the time of handshake required by the system. In addition, since it is needed to carry on the second training, the implementing procedure is relatively complicated.
The second method is to select a mode according to an uplink handshake signal.
The specific implementing procedure of the second method includes: at the handshake stage, the xTU-C which is responsible for mode selection estimates attenuation of the uplink signal sent from, the xTU-R according to the receiving level of uplink handshake signal and its transitting level, and deduces a length of a line and selects a corresponding operating mode according to preferred parameters of the known arts. For instance, the ADSL2+ is used within 2.8 km, the ADSL2 is used between 2.8 km and 3.8 km and the ADSL2 annex L is used if above 3.8 km.
Although the procedure of employing the second method to select an operating mode is simple and does not need to waste the time caused by the handshake procedure, the method is too coarse because it deduces the length of the line according to the uplink handshake signal and selects the operating mode according to experiences. As the attenuation of the uplink signal can not reliably express the attenuation of the downlink signal and further deduce the capacity, hence, this method of selecting an operating mode may fail to propose an optimal operation mode.